(1) Field of the Invention
The present invention relates to a multiplex control system having a plurality of controllers. More particularly, the present invention is concerned with a multiplex control system in which an object to be controlled, i.e., a controlled system, is controlled by means of a plurality of controllers with a high degree of reliabililty.
(2) Description of the Related Art
Hitherto, multiplexed switching of a plurality of controllers, in particular programmable controllers, is conducted by means of hardware in accordance with a switching logic prepared by the manufacturer. Thus, users are not allowed to freely set or renew the switching logic.
A data transfer system for a duplex computer system, forming such a switching system, has been described in Japanese Patent Unexamined Publication No. 57-86972. This system can be realized as a duplex system having a backup control system or a triplex system incorporating a multiplex control system which operates with a 2-out-of-3 rule. In each case, however, the switching is performed by hardware arranged exclusively for this purpose so that the switching logic is fixed, i.e., the user is not allowed to freely set or alter the switching logic. For instance, in the 2-out-of-3 type system, the operation cannot be performed unless two out of three systems are safe. Thus, it has been difficult to operate the system with a single system at the time of start-up or maintenance of the whole system.